The present invention relates to systems, such a subscription television systems, wherein it is desired that the audio signal being transmitted be encoded so that it cannot be properly received without activation of a decoding mechanism. In known systems of this type, as, for example, the system disclosed in U.S. Pat. No. 3,824,332 (Horowitz), the audio program signals are encoded by modulation on a suppressed carrier centered above the audio range. This system has the disadvantage that the aural transmitter must be operated above the normal audio range. This requires pre-emphasis for the high frequencies and also requires a larger dynamic range for the transmitter. Further, the spectrum space occupied by the transmitted signal corresponds to the space required for both side-bands. At the receiving end, special detecting equipment is required since the signal is not suitable for utilization by the audio detectors in the standard television receiver. The sound in these systems thus emerges out of the box housing the decoder rather than out of the television set.